One type of a conventional apparatus for FM-modulation of digital signals comprises a signal separating circuit for separating an input digital data signal into two digital data signals, a waveform shaping circuit for converting the two digital data signals to a reference phase signal (defined "I signal.revreaction. hereinafter) and an orthogonal phase signal (defined "Q signal.revreaction. hereinafter) in accordance with the waveform shaping thereof, and an orthogonal modulation circuit for modulation of a carrier wave signal in accordance with the I and Q signals to generate a digital FM-modulation signal.
In operation, the input digital data signal is separated into the two digital data signals by the signal separating circuit, and is simultaneously subject to logic processing of the signal in which the differential conversion, the prohibition processing for a specified data input, etc. are carried out. The two digital data signals of a rectangle waveform are shaped to realize MSK modulation in the subsequent orthogonal modulation by a filter of the waveform shaping circuit, so that the I and Q signals are obtained therein. In the orthogonal modulation circuit, the carrier wave signal is modulated by the I signal, while the carrier wave signal which is phase-shifted by .pi./2 is modulated by the Q signal, and the respective modulated signals are combined to provide the digital FM-modulation signal.
However, the conventional apparatus for modulation of digital data signals has a disadvantage in that a content of a received digital FM-modulation signal may be inverted dependent on the arrangement of a local signal in an RF band, when the received digital FM-modulation signal is demodulated, although the detailed reasons will be explained later.